Project Summary Chronic lymphocytic leukemia (CLL) is the most common leukemia in adults. According to the National Cancer Institute, approximately 16,060 patients will be diagnosed with CLL and approximately 4,580 patients will die of CLL in the US in 2012. At the Moffitt Cancer Center in Tampa, Florida, we see about 200 new CLL patients each year. Although CLL initially responds to chemotherapy, the relapsed CLL occurs at a high rate and acquires chemoresistance. Therefore, CLL is still incurable. We propose to identify the critical mechanisms that CLL cells rely on for their survival and target one such mechanism to block or decelerate aggressive progression of CLL. Robust B cell receptor (BCR) signal transduction was suggested to be responsible for the rapid proliferation of CLL cells, leading to aggressive progression of disease. Although protein antigen has been suggested to trigger the growth and proliferation of CLL cells, this concept that antigen can drive malignant progression of CLL has not been recapitulated in an animal model. Therefore, we have created a novel antigen-specific CLL mouse model, in which we use a desired antigen to activate the BCR and drive malignant progression of CLL. When a B cell is stimulated by its cognate antigen, activation of the endoplasmic reticulum (ER) stress response occurs to support B cell growth and proliferation. We hypothesize that engagement of the BCR by protein antigen can activate the ER stress response in CLL cells to promote malignant progression of CLL in vivo. We have shown that the inositol-requiring enzyme-1 (IRE-1)/X-box- binding protein-1 (XBP-1) pathway of the ER stress response is critical for the survival of CLL cells. Blocking the expression of XBP-1 by a novel small-molecule chemical inhibitor induces apoptosis in CLL cells in culture. To better understand the role of the IRE-1/XBP-1 pathway in the progression of CLL, we have genetically deleted the XBP-1 gene from our novel antigen-specific CLL mouse model. Using this innovative mouse model together with our novel small-molecule inhibitors, we will investigate the mechanisms by which blocking the IRE-1/XBP-1 pathway can decelerate antigen-induced aggressive progression of CLL in vivo. In cells with genetic deletion or chemical knockdown of XBP-1, we observed that IRE-1 is expressed at an elevated level and acquires a unique phosphorylation pattern. IRE-1 has been known for its roles in promoting cell survival and inducing apoptosis, but it is still unclear how IRE-1 accomplishes these two seemingly opposing tasks. We hypothesize that differential phosphorylation patterns may allow IRE-1 to associate with different interacting partners to carry out its functions in promoting survival or inducing apoptosis in CLL. Our goals in this proposal are summarized by two aims: 1) Target the IRE-1/XBP-1 pathway in antigen-induced aggressive progression of CLL in vivo; 2) Identify and investigate proteins that interact with IRE-1 to further understand how targeting the IRE-1/XBP-1 pathway can lead to stalled progression of CLL. Our goal is to establish the ER stress response as a useful target for the treatment of CLL.